The most commonly employed dye image--i.e., color--photographic elements are those containing one or more negative working radiation sensitive silver halide emulsion layers. Their widespread use is attributable to the excellent quality dye images they are capable of producing and to their high speed, allowing them to be employed in hand-held cameras under a variety of lighting conditions.
Nevertheless, negative working silver halide color photographic elements exhibit a significant limitation in terms of viewing the photographic image. Imagewise exposure of the negative working silver halide emulsion layer does not produce an immediately viewable dye image. Exposure produces an invisible latent image in the silver halide emulsion. Additional steps are required to produce a viewable dye image. Historically this has meant removing the photographic element from the camera, processing in aqueous solutions, and obtaining a first dye image. The first dye image obtained is a negative image, and a second exposure through the negative dye image of an additional photographic element and processing thereof is required to produce a viewable positive dye image of the subject initially photographed. (It is also possible with element or process modifications to produce a positive dye image directly in the photographic element which is imagewise exposed.)
Image transfer photography has made it possible to reduce the delay between imagewise exposure and obtaining a viewable dye image. Immediately after imagewise exposing the negative working radiation sensitive silver halide emulsion layer or layers, a processing solution can be brought into contact therewith. As silver halide development occurs, a transferred dye image can be formed in a receiving layer for viewing. In this way, visual access to the photographic image can occur in minutes or even seconds.
Still, though measured in seconds, the delay in providing visual access remains an important limitation in silver halide image transfer photography. Subject opportunities can be fleeting, and the photographer needs as nearly an instantaneous visual verification of an acceptable photographic image as can be offered.
Although image transfer has reduced the time required for image access in silver halide photography, this advantage has not been achieved without other sacrifices. One significant long term concern of image transfer photography relates to consumption of silver. Multicolor silver halide photographic elements which are conventionally processed and dye image transfer film units both employ relatively high silver coverages to obtain maximum photographic speed. Typically about 1000 milligrams of silver per square meter is required to form each of the blue, green, and red exposure records. In conventionally processed multicolor photographic elements the image produced contains no silver and all of the silver present in the photographic element is, in theory, recoverable. On the other hand, in image transfer photography silver is seldom recovered, and in integral format image transfer film units all of the silver remains with the photographic film units forming the viewable image.
Another disadvantage, inherent in dye image transfer photography, is the reduction in image sharpness attributable to dye diffusion. As image dye diffuses from the silver halide emulsion layer or an adjacent dye releasing layer, diffusion occurs both in the direction of the receiving layer and laterally, leading to image spreading and loss of sharpness. Sharpness can be improved by decreasing the length of the diffusion path to the receiving layer. This is controlled by the number and thickness of the layers to be traversed. Unfortunately, the minimum thickness of the silver halide emulsion layers is limited by the size of the silver halide grains and the weight ratio of gelatin to silver halide. Further, in multicolor image transfer film units employing three superimposed dye providing layer units, intervening dye providing layer units and separating interlayers must be penetrated by diffusing dyes migrating to the receiving layer.
Another consideration that arises in dye image transfer photography is image density variance as a function of temperature differences. Since subject opportunities are presented under a variety of temperature conditions and since the primary advantage of image transfer photography is ready image access, it follows that the ability of dye image transfer film units to produce acceptable dye images at a variety of temperatures is also important. Image transfer photography is much different than conventional photography in this respect, since in the latter processing is rarely undertaken without control of temperature.
In considering the characteristics of dye image transfer film units employing negative working silver halide emulsions, whether the dye image providing material is initially mobile or immobile is of importance as well as whether the dye image providing material is positive working or negative working. In general positive working dye image providing materials are most commonly employed with negative working silver halide emulsions, since, in the absence of uncommon reversal techniques, this results in a viewable positive transferred dye image. Although typically more complex in their imaging mechanisms, initially immobile positive working dye image providing materials are generally preferred over initially mobile positive working dye image providing materials, since image dye transfer can be more easily controlled. For specialized applications, such as when a negative transferred dye image is required or the subject being photographed is itself a negative image, negative working dye image providing materials of the type commonly employed in combination with direct positive silver halide emulsions to produce positive transferred dye images can be employed in combination with negative working silver halide emulsions. Turning to negative working dye image providing materials, the initially immobile dye image providing materials that have been developed for use with direct positive silver halide emulsions tend to yield high minimum densities and low image discrimination when employed in combination with camera speed negative working silver halide emulsions.
Silver halide image transfer film units are generally well known in the art of photography and require no detailed description. Broad discussions of image transfer elements and processes (including process solutions) can be found in Chapter 12, "One Step Photography", Neblette's Handbook of Photography and Reprography Materials, Processes and Systems, 7th Ed. (1977), in Chapter 16, "Diffusion Transfer and Monobaths", T. H. James, The Theory of the Photographic Process, 4th Ed. (1977), and A. Rott and E. Weyde, Photographic Silver Halide Diffusion Processes, Focal Press, (1972). Patents relating to silver halide image transfer are collected in U.S. Patent and Trademark Office Class 430 RADIATION IMAGERY CHEMISTRY--PROCESS, COMPOSITION OR PRODUCT, subclasses 199 through 255.
A great variety of regular and irregular grain shapes have been observed in silver halide photographic emulsions intended for imaging applictions. Regular grains are often cubic or octahedral. Grain edges can exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical or near spherical thick platelets, as described, for example by Land U.S. Pat. No. 3,894,871 and Zelikman and Levi Making and Coating Photographic Emulsions, Focal Press, 1964, page 223. Rods and tabular grains in varied portions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has been varied during precipitation, as occurs, for example in single-jet precipitations.
Tabular grains (those areally extended in two dimensions as compared to their thickness) have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. A discussion of tabular bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Silver Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Tabular silver bromide emulsions are discussed by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125. Sulfur sensitized tabular grain silver bromide emulsions having an average aspect ratio of from about 5 to 7:1 wherein the tabular grains account for greater than 50% of the projected area of the total grain population were incorporated in a direct X-ray radiographic product, No Screen X-Ray Code 5133, sold by Eastman Kodak Company from 1937 until the 1950's. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographic Emulsions", Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
Bogg, Lewis, and Maternaghan have recently published specific procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounded by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of {100} crystal faces. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994 and 4,184,877, U.K. Pat. No. 1,570,581, and German OLS publications Nos. 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromoiodide containing 40 mole percent iodide also contains 60 mole percent bromide.) Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. Bogg specifically reports an upper limit on aspect ratios of 7:1, and, from the very low aspect ratios obtained by the examples, the 7:1 aspect ratio appears unrealistically high. It appears from repeating examples and viewing the photomicrographs published that the aspect ratios realized by Lewis and Maternaghan were also less than 7:1.
Class 430, cited above, subclass 217 (Silver halide identified grain, identified emulsion binder other than nominal gelatin, or identified sensitizer or identified desensitizer containing) contains a collection of patents directed to silver halide image transfer photography, many of which disclose specific silver halide grain structures.
Wilgus and Haefner discloses high aspect ratio silver bromoiodide emulsions and a process for their preparation. U.S. Pat. No. 4,439,520 discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions.
Daubendiek and Strong U.S. Pat. No. 4,414,310 discloses an improvement on the processes of Maternaghan whereby high aspect ratio tabular grain silver bromoiodide emulsions can be prepared.
Abbott and Jones U.S. Pat. No. 4,425,425 discloses the use of high aspet ratio tabular grain silver halide emulsions in radiographic elements coated on both major surfaces of a radiation transmitting support to control crossover.
Wey, U.S. Pat. No. 4,399,215 discloses a process of preparing tabular silver chloride grains which are substantially internally free of both silver bromide and silver iodide. The emulsions have an average aspect ratio of greater than 8:1.
Solberg, Piggin, and Wilgus U.S. Pat. No. 4,433,048 discloses high aspect ratio tabular grain silver bromoiodide emulsions wherein a higher concentration of iodide is present in an annular region than in a central region of the tabular grains.
Dickerson U.S. Pat. No. 4,414,304 discloses producing silver images of high covering power by employing photographic elements containing forehardened high aspect ratio tabular grain silver halide emulsions.
Mignot U.S. Pat. No. 4,386,156 discloses high aspect ratio tabular grain silver bromide emulsions wherein the tabular grains are square or rectangular in projected area.